D.A.
My son is 2.5 and is fully potty-trained. I can tell you what worked for me -- bribery! :) We do not allow candy too much in our house, so when the kids do get it, it's a BIG TREAT. So we put peppermints (individually wrapped) in the bathroom so he could SEE what he was working for, and of course showered him with lots of praise when he did well, followed by that ever-important piece of candy. Peppermints just happen to be what he loves -- find out what your son loves and take it from there. Who knows - it just might work! Also, I should note that my son will not poop on the big potty, only his training potty. I'm not sure why, but as long as it's not in his pants I'm okay with that! Good luck!